This invention relates to flexible magnetic recording media which comprise a polyester substrate which has been coated with a magnetizable layer. More particularly, the present invention relates to a polyester film which has been coated with an adhesion-promoting primer layer prior to being coated with a coating composition comprising ferromagnetic particles dispersed within a polymeric binder.
Flexible magnetic recording media, such as audio, video, and computer tape, is typically manufactured by coating biaxially oriented polyester film with a solution of ferromagnetic particles, such as iron oxide or chromium oxide particles, and a polyurethane binder pre-polymer which has been dissolved in a strong solvent such as tetrahydrofuran, methylethylketone, methylisobutylketone, or cyclohexanone. A strong solvent is typically used in order to render the polyester surface more receptive to the ferromagnetic coating. The solvent is typically removed during drying of the polyurethane pre-polymer.
The binders typically employed to manufacture flexible magnetic recording media, commonly referred to as "magnetic tape", are polyurethanes, polyvinyls, and polyurethane/polyvinyl graft copolymers. Other polymeric additives may be included to achieve the optimum hardness, flexibility and adhesion. In addition, lubricants, dispersants, and curing agents are typically included in the coating formulation.
Magnetic tape must possess exacting physical, chemical and magnetic properties. Physical requirements include an acceptable coefficient of friction, high modulus of elasticity and tensile strength, and good abrasion resistance. The polymeric binder must be adherent to the polyester substrate, be chemically compatible with the ferromagnetic particles, and exhibit long-term stability. Finally, the magnetic tape must have a high data storage capacity.
In recent years magnetic tape manufacturers have sought to increase the information density and overall quality of flexible magnetic recording media in order to accommodate recent improvements in recording format technology such as perpendicular recording, digital recording and decreasing track width. Manufacturers have demanded improvements in polyester substrates, particularly thinner films with enhanced mechanical properties, smoother surfaces and better adhesion to the ferromagnetic coating. The industry has also sought to develop thinner, smoother ferromagnetic coatings which incorporate smaller particles and which possess higher coercivity.
As part of this development effort, the American National Standard Institute ("ANSI") has issued a proposed American National Standard for coating adhesion to the tape substrate (ANSI Project #488, Third Draft, April 19, 1985). The proposed standard requires that the force required to peel a ferromagnetic coating from the tape substrate be greater or equal to 0.12N/mm (0.69 lbs/inch) of tape width. Although at least one magnetic tape manufacturer has produced magnetic tape which exhibits a peel strength greater than 0.69 lbs/inch, several commercially available magnetic tapes cannot meet this proposed ANSI standard.
One approach to ehhance adhesion is to subject the film surface to an electric corona discharge treatment prior to coating the film with the ferromagnetic coating. This approach is not favored due to the fire and explosion hazards associated with the use of high voltage electrical equipment in the presence of volatile organic solvents such as those employed in the production of flexible magnetic recording media.
Another approach has been to interpose an adhesive or primer layer between the polyester film and the ferromagnetic coating. For example, U.S. Pat. No. 3,215,554 discloses vinylidene chloride/acrylonitrile copolymers as a primer layer, while U.S. Pat. No. 3,387,995 suggests a polycondensate of terephthalic acid, isophthalic acid and ethylene glycol. U.S. Pat. No. 3,661,874 teaches the use of an aminized reaction product of an epoxidized polybutadiene will enhance adhesion. Finally, U.S. Pat. No. 4,210,703 discloses a cationically polymerized epoxy resin which improves adhesion and also prevents extraction of low molecular weight oligomers from the polyester substrate by the strong solvents employed in magnetic tape manufacture.